Flame arrestors are passive devices designed to prevent propagation of gas flames through pipelines. A flame arrestor incorporates a permeable barrier known as an element which is usually a matrix of metallic, ceramic or mixed materials that define a permeable barrier containing narrow channels. An element removes heat and free radicals from a flame at a rate which is fast enough to quench the flame and to prevent reignition of the hot gas on the protected side (downstream relative to the direction of flame propagation along a pipe) of the arrestor.
A flame arrestor is located in a pipeline carrying a flammable gas, and the design of a flame arrestor can vary greatly depending upon application, location and use conditions. For example, a best design for a particular installation may take into account flow resistance, maintainability and cost.
For purposes of evaluating efficacy of a particular flame arrestor for particular flame arrestor applications, various testing protocols have been developed that aim to address most adverse conditions encountered. In, for example, the case of marine vapor control systems in the United States, the testing and application of flame arrestors is regulated by the U.S. Coast Guard.
A flame arrestor can be used to arrest deflagrations and detonations. A deflagration is a combustion wave propagating at less than the speed of sound as measured in unburned gas immediately ahead of the flame front. Flame speed relative to unburned gas is typically 10–100 m/s (meters per second), but, owing to expansion of hot gas behind the flame, several hundred meters per second may be achieved relative to a pipe wall. Although the pressure peak coincides with the flame front, a marked pressure rise precedes it, so that the unburned gas is compressed as the deflagration proceeds, depending upon flame speed and available vent paths. The precompression of gas ahead of the flame front establishes the gas conditions in the arrestor when the flame enters it and hence affects both the arrestment process and the maximum pressure generated in the arrestor body.
As a deflagration travels through piping, its speed increases due to flow-induced turbulence and compressive heating of unburned gas ahead of the flame front. At a flame speed approaching sonic velocity, a deflagration-to-detonation transition (DDT) can occur with associated abnormally high velocities and pressures. At the instant of transition, a transient state of overdriven detonation is achieved and persists for a few pipe diameters. After the decay of such conditions, a stable detonation state is attained. A detonation is a combustion-driven shock wave propagating at the speed of sound, as measured in the burned gas immediately behind the flame front. Stable detonations propagate at sonic velocities relative to an external fixed point. A wave is sustained by chemical energy released by shock compression and ignition of unreacted gas. The flame front is coupled in space and time with the shock front, with no significant pressure rise ahead of the shock front.
The high velocities and pressures associated with detonations require special element design to quench the high-velocity flames plus superior arrestor construction to withstand the associated impulse loading. In practice, this entails narrower and/or longer element channels plus bracing of the element facing.
The problem of flame arrestment, either of deflagrations or detonations, depends on the properties of the gas mixture plus the initial pressure. Since gas mixture combustion properties cannot be quantified for direct use in flame arrester selection, flame arrester performance must be demonstrated by realistic testing.
A severe deflagration arrestment test involves placing a restricting orifice behind the arrestor (that is, upstream relative to the direction of wave propagation). Such a restriction produces a so-called reflection wave that travels back to the flame arrestor from the restriction and increases the degree of precompression. Such “restricted end” deflagration testing constitutes a severe deflagration arrestment test, yet such testing is believed to represent an operating environment that can exist in fact from various conditions, such as when, for example, a closed or partially closed valve in a pipe is located upstream from a functioning arrestor in the pipeline. Such testing has demonstrated that arrestors capable of stopping even overdriven detonations may fail under restricted end deflagration test conditions.
The art of flame arrestors needs improved apparatus and methods for achieving arrestment in environments where reflection waves can be generated upstream relative to the direction of wave propagation and be propagated back to a flame arrestor. The present invention provides such improvements.